None
Not applicable.
The present invention generally relates to adhesives for joining wood products, and more particularly to two-part polyurethane adhesives modified with an aqueous emulsion.
Glued wood products have been traditionally used in this country in a variety of applications. The adhesives for such bonded or laminated wood products conventionally are based on phenol-formaldehyde, urea-formaldehyde, polyvinyl acetate, resorcinol-formaldehyde, polymeric diphenylmethane diisocyanate (MDI), and hot met adhesives. The glued wood products include plywood, particleboard, oriented strand board (OSB), medium density fiberboard (MDF), laminated veneer lumber (LVL), laminated beams, and a variety of other engineered wood products. Among them, laminated beams, I-beams, LVL, and a variety of engineered wood products are used for structural wood applications. Generally, these engineered wood products require an initial finger jointing of short pieces of wood or parallel laminated veneers (PLV) before they can be constructed into long and/or thick beams or lumbers and in the case of I-Beams an adhesive is required to assemble the I-Beam itself. A fast curing structural adhesive is required to manufacture I-Beams in both the web-to-flange joint as well as the butt joint between web pieces. Consequently, it is important that, in the finger-jointed area and the I-Beam bonding areas, the adhesive have good strength, structural adhesive properties, and cure quickly. For present purposes, all of the foregoing products are known as xe2x80x9claminated wood products.xe2x80x9d
At present, phenol-resorcinol-formaldehyde (PRF) is widely used in industry for finger joint applications. When adhesive is applied to the fingers, the finger jointed wood or PLV is crowded together using an end pressure until a xe2x80x9ctip gapxe2x80x9d of 1-40 mils is achieved. Its is essential that the fingers do not xe2x80x9cbottom out.xe2x80x9d The finger joints then are moved into a curing zone where hot platens or dielectric plates are used to cure the finger joints under heat or radio frequency and pressure for typically less than 30 seconds and then the joints are removed away from the curing zone. The adhesives must be able to fill the gaps or voids between the fingers when curing is complete in order for the product to exhibit good strength and a smooth appearance.
One the other hand, the speed of curing must be fast under these conditions in order to hold the finger-jointed pieces together for further processing, such as beam lamination and I-joint assembly. This is especially true in a high-speed commercial finger jointing process. Generally, adhesives with high solids and fast curing profiles are regarded as appropriate for such an application.
It should be mentioned also that two-part PRF and melamine-formaldehyde (MF) adhesives generally are used in industry as adhesives in preparation of radio frequency (RF) cured finger joint assemblies. Both of these adhesives are highly polar, which makes them respond well to RF curing. RF curing drives the bondline temperature sufficiently high to permit the MF or PRF to condense quickly. Due to the rigid ring structure of both PRF and MF, these adhesives cure to give a high Tg, rigid, three-dimensional network in a short time; thus, producing acceptable immediate handling strength (proof-loading).
Heretofore, U.S. Pat. No. 3,931,088 proposes an aqueous adhesive composition of an aqueous solution of polyvinyl alcohol, an aqueous emulsion of vinyl acetate polymer or an aqueous latex of a butadiene polymer in combination with a hydrophobic solution of an isocyanate compound. RE 34,093 proposes an aqueous adhesive composed of an aqueous emulsion of a polymer or interpolymer of one or more of vinyl monomers in combination with an aqueous emulsion of a polyisocyanate, which has been at least partially reacted with a C6 to C30 alkanol.
One aspect of the present invention is an adhesive composition formed from two packs. One pack includes an isocyanate-functional prepolymer formed by reacting a polyisocyanate with an alcohol. The isocyanate equivalents from the polyisocyanate exceed the hydroxyl equivalents from the alcohol. The second pack is an aqueous polymer emulsion. These two packs are combined to form the novel adhesive composition. xe2x80x9cAlcoholxe2x80x9d for present purposes includes polyols.
Another aspect of the invention is a method for joining together two surfaces of wood products by application of an adhesive thereto. The first step in this method is to mist with water the wood product surfaces to be joined. An adhesive composition then is applied to the misted wood product surfaces. Alternatively, the water misting can be replaced with an aqueous emulsion being part of the adhesive composition. Finally, the adhesive composition is cured. This method works best when the moisture content (MC) of the wood products to be joined is less than about 10%.
Advantages of the present invention include the ability of the inventive adhesive to qualify as a structural adhesive. Another advantage is the ability of the inventive adhesive to qualify as an exterior structural adhesive for engineered wood applications. These and other advantage will be readily apparent to those skilled in the art based on the disclosure set forth herein.